1. Field of the Invention
The present invention relates to a synthetic resin pallet and more particularly to a pallet for a forklift used for carrying, moving and storing goods.
2. Description of the Related Art
A pallet for a forklift, which is composed of a synthetic resin, has hitherto been manufactured and used. There are a various types of synthetic resin pallets. One method of forming the synthetic resin pallet may be exemplified by a method of forming top and bottom pallet constructive members divided by two in a height wise direction by injection molding, then setting leg (block) constructive members in these pallet constructive members facing to each other, fitting them to each other, and integrally welding the fitted portions thereof by heat.
Each of the top and bottom pallet constructive members includes a plate member, leg constructive members provided in proper positions on an internal surface of this plate member, and reinforced ribs provided in lattice intersecting lengthwise and crosswise on the internal surface of the plate member. The "plate member" is a high-order conceptual term, and literally implies a plate-like portion having a comparatively broad surface. Especially the plate member designed for loading a goods on its surface is referred to as a loading surface, i.e., a deck board.
Accordingly, in the case of the synthetic resin pallet of which both surfaces are used, these two plate members are defined as the deck boards. In the case of the synthetic resin pallet of which one single surface is used, the plate member for loading the goods is in particular called the deck board, while the other surface is simply termed the plate member.
The reinforced ribs are formed in lattice on the internal surface of the plate member of each of the pallet constructive members. The reinforced rib is well known as what is extremely effective in terms of enhancing a flexural strength of the plate member and preventing a flexure from occurring in the plate member, especially the deck board when a load is carried and moved by a forklift.
The reinforced rib described above has an advantage of enhancing the flexural strength of the plate member of each of the pallet constructive members, and also has an advantage of increasing a strength against a fracture in the vicinity of a fork insertion hole of the synthetic resin pallet, which might easily be fractured by an impingement of the fork tines. Accordingly, the reinforced ribs are formed over the entire surface of the plate member of each of the pallet constructive members constituting the synthetic resin pallet.
Incidentally, the goods are stored for a certain period of time in an automated warehouse as the case may be. In that case, the synthetic resin pallets loaded with the goods are stored while being supported by a rack device in the automated warehouse. The automated warehouse generally has a mechanism for picking up the synthetic resin pallet from under by a stacker crane, and therefore in great majority of cases takes a "two-point support rack structure" for supporting the synthetic resin pallet at only right-and-left ends thereof.
In the pallet formed of the synthetic resin, however, if a considerable time has elapsed in a state where a load is applied to the pallet, a fatigue phenomenon known as a "creep", i.e., a flexure occurs. The occurrence of this flexure will be explained in much greater details. As shown in FIG. 15, the flexure occurs when supporting the two ends of the pallet in the state where the goods are placed on the synthetic resin pallet. In that case, as indicated by arrowheads 28a, 28b in FIG. 15, the surface of the deck board is compressed by the flexure, while the plate member on the underside is stretched, thus deforming the synthetic resin pallet.
The occurrence of this flexure is a conspicuous phenomenon with macromolecules. Therefore, the creep, viz., the flexure occurred in the synthetic resin pallet can not be eliminated in terms of its material, and hence the conventional synthetic resin pallet has been contrived to reduce the creep deflection by physical means.
As one example thereof, there has been known a method of enhancing strength against bending force acting in a direction right-angled to the surface of the deck board by such a structure that a rigid body such as elongate steel bars or fiber reinforced plastics bars is embedded extending in bilateral directions of the pallet inwardly of the deck board of the synthetic resin pallet. This method, however, has a defect that a weight of the synthetic resin pallet is extremely heavy, and also presents a problem that a crack is easily produced in the resinous portion peripheral to the steel core embedded therein.
Further, when the rigid body such as the steel core is so disposed as to be embedded in the synthetic resin pallet, a problem is that a deviation and an air gap might occur due to a repetitive stress between the rigid body and the peripheral resinous portion, and eventually the crack occurs around the rigid-body-embedded portion in the synthetic resin pallet. Those problems arise because the synthetic resin pallet by nature bends or deflects to some extent by the load as described above, and nevertheless such a reinforced structure that the rigid body embedded in the pallet receives almost all of the loads upon the synthetic resin pallet.
Further, there has been examined such a proposal that the synthetic resin pallet is molded by mixing a reinforced fiber such as a glass fiber in the synthetic resin material as a main material of the synthetic resin pallet, is thus given a high strength against the flexure itself. If the fiber reinforced material is mixed in the synthetic resin pallet itself, there is a defect wherein the weight of the whole pallet becomes heavy, and besides a pallet decreases in its impact strength.